1. Field of the Invention
The present invention relates to display devices that are AC driven, such as active-matrix liquid crystal display devices for example. More specifically, the present invention relates to display devices wherein a multitude of video signal lines for transmitting video signals to a plurality of pixel formation portions for forming an image to be displayed are grouped together to a plurality of video signal line groups, taking a plurality of (for example two) video signal lines as one group, and a video signal is outputted from a driving circuit by time division to each of the video signal line groups.
2. Background of the Invention
In recent years, there have been tremendous advances in achieving a higher level of detail for images displayed on display devices. Therefore, in display devices requiring a plurality of signal lines (column electrodes or row electrodes) corresponding to the resolution of the image to be displayed, as in an active matrix liquid crystal display device for example, the number of signal lines (electrodes) per unit length becomes very large, as the level of detail of the displayed image increases. As a result, when mounting the driving circuit applying signals to those signal lines, the pitch of the connection between the output terminals of the driving circuit and the display panel signal lines (referred to as “connection pitch” below) becomes extremely small. This trend to a narrower connection pitch that is brought about by the increased level of detail of the displayed image is particularly striking in the connection portions between the video signal lines (column electrodes) and their driving circuit (referred to as “column electrode driving circuit,” “data line driving circuit” or “video signal line driving circuit”) in the case of a color display device in which the neighboring three pixels of R (red), G (green) and B (blue) are taken as display units, as in a color liquid crystal display device.
In order to solve this problem, a liquid crystal display device has been proposed, in which two or more video signal lines (for example the three video signal lines corresponding to three neighboring R, G and B pixels) are grouped together, one output terminal of the video signal line driving circuit is assigned to the plurality of video signal lines constituting each group, and in one horizontal scanning period of the image display, video signals are applied by time division to all video signal lines within each group (see JP H6-138851A, for example).
FIG. 2A schematically shows the configuration of the connection between the video signal lines and the driving circuit thereof (referred to as “video signal line driving circuit” in the following) in an active matrix-type liquid crystal display device using this scheme (referred to as “video signal line time-division driving scheme” in the following). In the example shown in FIG. 2A, two video signal lines Ls each are grouped into one group, and each of the video signal line groups corresponds to one of the output terminals TS1, TS2, TS3, . . . of the video signal line driving circuit 300. One selector switch is disposed between each of the output terminals TS1, TS2, TS3, . . . of the video signal line driving circuit 300 and the two video signal lines of the group corresponding to that output terminal. Each of the selector switches is made of two neighboring analog switches SWi and SWi+1 (i=1, 3, 5, . . . ) of the analog switches SW1, SW2, SW3, . . . that are each provided for one of the video signal lines Ls, and one side of each of the analog switches SW1, SW2, SW3, . . . is connected to one of the video signal lines Ls. The other sides of the two analog switches SWi and SWi+1 constituting each selector switch are connected to one another, and are connected to the output terminal TSj (j=1, 2, 3 . . . ) of the video signal line driving circuit 300 corresponding to that selector switch. These selector switches may be realized as analog switches by thin-film transistors (TFTs) formed on the liquid crystal panel substrate of the display device, for example.
FIGS. 4A to 4D are timing charts showing the scanning signals G1, G2, G3, . . . in a liquid crystal display device of this video signal line time-division driving scheme and the control signal (referred to below as “switching control signal”) GS for the selector switches. Here, when the scanning signal Gk is at high level (H level), the k-th scanning signal line is selected, and when the scanning signal Gk is at low level (L level), the k-th scanning signal line is unselected (k=1, 2, 3, . . . ). Moreover, when the switching control signal GS is at H level, the selector switches connect each of the output terminals TSj (j=1, 2, 3, . . . ) of the video signal line driving circuit 300 to the left one of the two corresponding video signal lines, and when the switching control signal GS is at L level, the selector switches connect each of the output terminals TSj (j=1, 2, 3, . . . ) of the video signal line driving circuit 300 to the right one of the two corresponding video signal lines. As shown in FIG. 4D, in this liquid crystal display device, in one horizontal scanning period, that is, in the period during which one scanning signal line is selected, the video signal line connected to each of the output terminals TSj is switched, and each of the video signals from the video signal line driving circuit are applied to the left one of the two video signal lines constituting one group in the first half of the horizontal scanning period, and to the right one of the two video signal lines in the second half of the horizontal scanning period. Thus, each video signal line Ls is charged with the voltage of the video signal that is outputted from the output terminal TSj of the video signal line driving circuit 300 while the output terminal TSj is connected to that video signal line Ls, and that voltage value is written as a pixel value into the pixel formation portion Px corresponding to the intersection between that video signal line and the selected scanning signal line.
In liquid crystal display devices using this video signal line time-division driving scheme, the time that each video signal line is charged is shortened in accordance with the number of video signal lines constituting each group, that is, the number of time divisions due to the selector switches. If m is the number of time divisions, then the charge time of each video signal line is 1/m of that in an ordinary liquid crystal display device not using the video signal line time-division driving scheme (½ in the example shown in FIG. 2). However, by forming, on the liquid crystal panel substrate, selector switches with a time division number of m, it is possible to make the pitch of connection of the output terminals of the video signal line driving circuit and the video signal lines m times that of an ordinary liquid crystal display device. Moreover, with this configuration, if a video signal line driving circuit is used that is made of a plurality of integrated circuit chips (IC chips) to drive one liquid crystal panel, then the number of those chips can be decreased.
The advantages of providing selector switches on the display panel substrate and driving the video signal lines by time division as described above, that is, the advantages of the video signal line time-division driving scheme are widely known, and for this, a plurality of video signal lines that are adjacent like, for example, the three video signal lines transmitting video signals to the three neighboring R (red), G (green) and B (blue) pixels are grouped together. In ordinary liquid crystal display devices, AC driving is performed in order to prevent deterioration of the liquid crystal and to sustain the display quality. A typical AC driving scheme is the so-called dot-inversion driving scheme, in which the polarity of the voltage applied to the liquid crystal layer forming the pixel is inverted at each scanning signal line and at each video signal line (and also inverted at each frame). When the above-described conventional video signal line time-division driving scheme is employed in liquid crystal display devices using this dot-inversion driving scheme, then the number of output terminals of the video signal line driving circuit is reduced, but the power consumption per output of the video signal line driving circuit increases in accordance with the number of time divisions (the number of video signal lines per group). That is to say, if a video signal line time-division driving scheme with m time divisions is applied, then, according to a simple model, the power consumption P per output of the video signal line driving circuit can be expressed by the following equation:P∝m·f·c·V2  (1)where, f denotes the frequency, c denotes the load capacitance that is driven by the video signal line driving circuit, and V denotes the driving voltage.